In summer to the beginning of autumn, pine needles of pines with the pine wilt nematode disease begin to change their color. After about one month, the entire crowns turn red and the pines wilt and die. The pathogen of the pine nematode disease is Bursaphelenchus xylophilus (hereinafter sometimes referred to as pine wilt nematode), which is a kind of nematode. The pine wilt nematode is carried by a Japanese pine sawyer (Monochamus alternatus), which is a kind of longicorn beetles.
In about the middle of May to the end of July, Japanese pine sawyers emerge and get out of pines which suffered from the pine wilt nematode disease the preceding year. The Japanese pine sawyers carry thousands to tens of thousands of pine nematodes in their bodies or on the surface of their bodies, and eat sprigs of healthy pines for maturing (maturation feeding). At this time, the pine nematodes get out of the bodies of the pine sawyers, enter the pine bodies at the injured sites eaten by the Japanese pine sawyers and grow.
After 2 to 3 months, the pines entered by the pine wilt nematodes apparently exhibit wilting and the pine needles change their color. The trees whose pine needles begin to change their color are referred to as abnormal trees. The Japanese pine sawyers matured by the maturation feeding copulate, and then lay eggs in the abnormal trees. The abnormal trees die soon.
The larvae of the pine sawyers can not sometimes hatch in healthy pine trees because the eggs laid in healthy trees are sometimes covered with the resin of the pines. Even if the larvae hatch, they are covered with the resin during feeding under the bark, and die. Thus, the Japanese pine sawyers hardly grow in healthy trees. However, the eggs laid in the dead trees or abnormal trees, which are weakened by the invasion of the pine nematodes, are not covered with the resin, and the larvae hatch out of them and grow by feeding under the bark.
The sufficiently grown larvae of the Japanese pine sawyers make pupal rooms deep in the pines in the end of autumn to the beginning of winter to stay over winter and avoid the cold of winter.
When spring comes and the temperature rises, the larvae of the pine sawyers which have stayed in the pupal rooms become pupae. Soon, the pine sawyers emerge, grow to be imagoes and get out of the dead trees. At this time, the pine nematodes gathered around the pupal rooms transfer to the emerging pine sawyers, and are carried out of the dead trees with the pine sawyers.
While the pine sawyers with the pine nematodes eat healthy sprigs of healthy pines, the pine nematodes get out of the bodies of the pine sawyers and enter the bodies of the pines. Soon, the pines exhibit wilting, and the pine needles begin to change their color. Thus, wilting of pines by the pine nematode disease spreads one after another.
The following methods have been used to prevent wilting of pines by the pine nematode disease.
(a) To lower the density of Japanese pine sawyers:
Pine sawyers are exterminated before the Japanese pine sawyers emerge, mainly at the larval stage. The habitable place of the larvae of the Japanese pine sawyers is restricted to dead trees. The dead trees are felled, the bark is stripped from the trees, and the trees are destroyed by fire or treated with chemicals to exterminate the Japanese pine sawyers. The treatment with chemicals is carried out, for example, by sprinkling organic phosphorus insecticides or carbamate insecticides, or by fumigating with metam ammonium insecticides. Further, methods utilizing insect-parasitic microorganisms or predacious natural enemies or the like have been examined.
(b) Prevention of Japanese pine sawyers from maturation feeding:
Imagoes of pine sawyers can be prevented from maturation feeding to prevent infection of pine nematodes. For this, insecticides are sprinkled before the emergence of the pine sawyers. Insecticides are sprinkled from the ground or dropped from a helicopter.
(c) Prevention of wilting by treating pine trees with chemicals one by one:
Before the infection of the Japanese pine sawyers is likely to happen, chemicals with insecticidal activity are injected into the trunks of pine trees or sprinkled on the soil to exterminate the pine wilt nematodes directly. As the chemicals injected into the trunks of the trees, mesulfenfos, morantel tartrate and levamisole hydrochloride have been used. As the chemicals sprinkled on the soil, ethylthiometon, methomyl, aldicarb and the like were found to be effective but have not yet been used practically.
(d) Breeding of resistant pines:
Expression of induced resistance and the like by selective or combination breeding or with attenuated pine nematodes have been examined to make pines resistant to infection with pine wilt nematodes, and gradually provided results. However, it takes much time to establish the technology.
The above methods for preventing the pine nematode disease have both merits and demerits. The method by felling the dead trees, stripping the bark from the trees and destroying the trees by fire or treating the trees with chemicals to exterminate the Japanese pine sawyers is problematic because of shortage of labor necessary to carry out the method completely.
The method by sprinkle insecticides to prevent Japanese pine sawyers from maturation feeding is effective. However, this method must be carried out with various restrictions depending upon circumstances.
The method by treating pine trees with chemicals one by one has been carried out particularly in places where it is difficult to sprinkle insecticides, for example, shrines, Buddhist temples, parks, towns and the like. However, this method is problematic in terms of damage from the chemicals injected into the trunks of pine trees, stability of the effect, the duration and the like.
Thus, there is still a need for another method for preventing wilting of pines caused by the pine nematode disease.